Frankenscope? Let's see it!***be advised that NOTHING in this forum has been safety tested and you are reading and using these posts at your own peril. blah, blah, blah... dont mess around with your eyesight when it comes to solar astronomy. Use appropriate filtration at all times...

I'm trying to gather information on this subject as I have seen some really lovely truss refractor designs using internal ERFs instead of massive full aperture ERFs which are also extremely expensive. However, I'm curious what the difference is in heat blocking/rejection between a full aperture ERF and a smaller, deeper in the light cone internal smaller aperture ERF would be. For example, say you take a 6 inch or 8 inch refractor objective, F6 to F8 and put an 75~90mm aperture ERF on the inside somewhere that the light cone would fit through clearly, before the focuser. Either well before the focuser or on the nose of the focuser tube, but without cutting into the light cone. Does that work or is it inherently going to not block as much energy compared to simply being full aperture in front of the objective?

Hi Marty,
I thought the erf should be about 2/3 of the front lens. But please wait for others to react before you act on this advice. (-;
Or try to find a second hand Erf and put it before the lens.
Bart.

Marty, ask any planetary and lunar imagers - do they willing to put even a very weak source of heat inside their telescopes.

Any erf can't reflect all the unwanted energy and will absorb some heat and so, produce some air turbulence.
This were especially well seen with the Quark Chromosphere worked in a 150mm telescope without front ERF
or with just UV/IR small filter. Some areas on the images were like covered by veil and look less sharp after the
post processing.

"Solar H alpha activity is the most dynamic and compelling thing you can see in a telescope, so spend accordingly." (c) Bob Yoesle.

The classical ratio for Ha ERF is 2/3. It works pretty well with a 250 mm F/10 refractor.

For Ca K, I use a 1/3 ratio (150 mm F/7 refractor and 50 mm internal ERF). It started as a test. It looks fine until now, but I would not recommend for the moment. Definite conclusions would wait the end of the summer season.

My guess is that all ERF are not equivalent. For internal position, you have to forget about substrates absorbing energy (RG630, etc).

Bases on numerous PST mods over the years,I always recommend the internal ERF be >50% the clear aperture andplaced as close to the objective as possible.
I also highly recommend the Baader D-ERF over the alternatives.

Hello,
I just did a test with a 250mm f2800 lens.
The D-erf 75mm is about 500mm from the camera, the D-erf has a big advantage is to reject with a mirror effect all that is not Ha.
The test only a few minutes without the filter heats too much.
When the weather decides to put on a nice weather, I will leave the filter longer.
Paul

An ERF has no impact at all on contrast, except if it is of bad optical quality or bad uniformity.
The contrast of Ha images depends on the Ha filter (= blocking photospheric contribution), the optical quality of the telescope and optical set-up(including level of diffusion) and the quality of seeing.
I have both the Baader D-ERF (160 mm) and the AiryLab ERF ( 210 mm). Both are of excellent quality. This is proven by a lot of Ha images I took with the TOA 150 and the C8 EDgeHD, and also mesurements on optical bench.
My 160 D-ERF is L/9.6 P-V :
My 210 mm AiryLab is L/10 P-V.

This beeing said, I won't pretend there is no other good / excellent ERFs, since ... I have tested no others. ;-)

- First of all, these filters are interference filters. They don't absorb light, they reflects light (absoption is typically lower than 0.1%).
- Accordingly, a blue filter reflects yellow light (= green + red), a green filter reflects magenta (= blue + red) , a red filter reflects cyan light (= blue + green).
- So, it is not suprising that the reflected light is yellow :-)
- I can see that the reflected beam meets partly with the tube on the refractor. This is indeed very bad. It means that the reflected beam heats the tube (remember the tube is black), which in turns heats the air, which finally builds up convection. This might be an explanation of what you are experiencing. I am not sure at 100%, because I am not there at your side to check what is going on ;-)
- So, the first thing to do is to tilt the blue filter so that the reflected beam remains inside the aperture of the objective of your refractor. It should not be center on the objective (otherwise you might have reflection at the focus), but off-center.

This is a bit late but I have a warning about sub-aperture, internal D-ERFs:

There is a fiercely hot, refocused and unfiltered, reflected Solar image well in front of my 6" f/8 [120/10] objective.
I'm using a 90mm internal Baader D-ERF with PST H-a filtration in an old CR150HD donor.

Fortunately, my head has usually blocked the sun's light when I happened to glance into the objective while pointing at the Sun.
But an unwary hand waved in front of the objective would probably be burnt since it can't eclipse enough incoming sunlight.

A long dewshield might be a very good idea where the public has even "accidental" access to such an instrument.
An amateur solar observer, with children wandering about, should be very aware of this very real danger.
A small child's head would still allow annular sunlight to fall on a larger objective with potentially tragic consequences.
Most of a lens's collecting area is in the outer zones.

A full aperture D-ERF filter, fixed securely in front of the objective, would minimize the risk down to "normal strength" reflected sunlight.
The reflected glare would usually be enough to cause a person, or child, to desist from their "Darwin Awards" curiosity.

However, a reflected and re-focused image, from an internal D-ERF, might not give the "accidental" viewer any chance at all to withdraw before damaging their sight permanently.
And yes, you can easily set light to a strip of wood held a few inches in front of the objective on my 6" modified refractor!
Fortunately my telescope is very inaccessible and large enough to need a stepladder to reach the objective.

Logic suggests that the larger the internal D-ERF the further forward of the objective the Sun's re-focused image will be thrown.
I wouldn't be leaving stepladders anywhere near such an instrument when leaving it unattended.!

This is a bit late but I have a warning about sub-aperture, internal D-ERFs:

There is a fiercely hot, refocused and unfiltered, reflected Solar image well in front of my 6" f/8 [120/10] objective.
I'm using a 90mm internal Baader D-ERF with PST H-a filtration in an old CR150HD donor.

Fortunately, my head has usually blocked the sun's light when I happened to glance into the objective while pointing at the Sun.
But an unwary hand waved in front of the objective would probably be burnt since it can't eclipse enough incoming sunlight.

A long dewshield might be a very good idea where the public has even "accidental" access to such an instrument.
An amateur solar observer, with children wandering about, should be very aware of this very real danger.
A small child's head would still allow annular sunlight to fall on a larger objective with potentially tragic consequences.
Most of a lens's collecting area is in the outer zones.

A full aperture D-ERF filter, fixed securely in front of the objective, would minimize the risk down to "normal strength" reflected sunlight.
The reflected glare would usually be enough to cause a person, or child, to desist from their "Darwin Awards" curiosity.

However, a reflected and re-focused image, from an internal D-ERF, might not give the "accidental" viewer any chance at all to withdraw before damaging their sight permanently.
And yes, you can easily set light to a strip of wood held a few inches in front of the objective on my 6" modified refractor!
Fortunately my telescope is very inaccessible and large enough to need a stepladder to reach the objective.

Logic suggests that the larger the internal D-ERF the further forward of the objective the Sun's re-focused image will be thrown.
I wouldn't be leaving stepladders anywhere near such an instrument when leaving it unattended.!

Very good information to know! I have I think the same exact scope, an old CR150HD refractor, 6" F8. I've been using it mostly lately. While it works fine with the 2" UV/IR cut filter and my Quark, I'm definitely thinking of putting an ERF inside it. A full aperture ERF is simply too costly right now, and I'd rather put that money towards a larger ERF ultimately (saving towards a big SCT size ERF eventually). But an internal 75mm or 90mm is $350 and $500 and that's much more affordable to tide me over a few more years while continuing to image.

My imaging scopes are never around other people, just me, in my observatory, pointing at the sky. So I'm not worried about others. But, the safety tips are very appreciated!

May I ask how you installed your Baader 90mm ERF internal in that scope? Would love to see how you managed it. Images would be lovely!

I was thinking about getting a 90mm ERF ($500 new) and installing it on a frame that bolts onto an internal baffle surface in the scope as the holding cell basically. The big question is... how and where to cut the OTA to get access, or, can I simply put it in via taking the front element cell off and reaching down into the OTA and some how getting it in there.

My other concern is being able to get it back out. I like to image in CaK with this same instrument, but I think the Baader ERF would kill that, so I'd have to be able to remove the the internal ERF some how. I'm trying to find a way to make this modular....